This invention relates to a shaft seal made from polytetrafluoroethylene as the sealing element and so constructed as to be bidirectionally hydrodynamic.
Molded, elastomeric bidirectionally hydrodynamic seals are quite common. However, there has been a total lack of success in the manufacture of hydrodynamic seals having a polytetrafluoroethylene sealing element, principally because of the difficulty in working with this material. Until recently the only known method of obtaining a raised flute formation on a polytetrafluoroethylene hydrodynamic seal was to mold it. Recently, it has been found possible to cold form hydrodynamic features into polytetrafluoroethylene sealing elements, as shown in copending application Ser. No. 523,451, filed Nov. 13, 1974 (a continuation-in-part of Ser. No. 426,365, filed Dec. 19, 1973, now U.S. Pat. No. 3,929,341 and assigned to the assignee of the present invention. This new method has made possible the forming of close dimensional tolerance in the design features of such seals and has made possible manufacture of the seal of the present invention. This is not to say the present invention could not be constructed as a molded polytetrafluoroethylene seal element, but only that it is believed that consistently good results are assured with the new method of manufacture.
Thus, attempts to make hydrodynamic seals of polytetrafluoroethylene were until recently unsuccessful and have become successful only as a result of work involving the present inventor, employing processes which involve either machining a spiral groove or other hydrodynamic structure into the seal or by coining the structure into the seal. The co-pending application Ser. No. 426,373 filed Dec. 19, 1973, now U.S. Pat. No. 3,857,156, does the former, while application Ser. No. 523,451, now U.S. Pat. No. 3,929,341, does the latter, and experience has shown that this latter is a practical method of manufacture of hydrodynamic polytetrafluoroethylene seals. In other words, a flat washer is cold-formed with the aid of a die under considerable pressure into a shape enabling hydrodynamic action. Up to now, however, such hydrodynamic structures gave hydrodynamic action in only one direction of shaft rotation.
Another type of polytetrafluoroethylene seal which has substantially the same foot pattern and functions hydrodynamically in accordance with the same sealing principle is shown in U.S. Pat. No. 3,801,114, granted Apr. 2, 1974, wherein bidirectional hydrodynamic action is achieved by employing three washers or annular wafers of polytetrafluoroethylene for each seal which washers are arranged concentrically side by side. The washers constitute a static seal lip, a set of hydrodynamic flutes, and a second but discontinuous seal lip, respectively. Such a seal performs well, but only when the respective washers are assembled absolutely concentrically, and this may be difficult to achieve on a production basis.
An object of the present invention is to provide a method for manufacturing a shaft seal in which the sealing element is polytetrafluoroethylene and in which that sealing element is so made as to be bidirectionally hydrodynamic. It is an important feature of the invention that the finished shaft seal employs only a single annular wafer or washer of polytetrafluoroethylene and not a plurality of such washers or wafers and yet is bidirectionally hydrodynamic.